Molded plastic parts have become increasingly popular, due at least in significant part to their low production cost. Hence, often, one or more components of an assembly are formed of molded plastic. In assembling a final product it is often necessary to fasten the molded plastic parts to other components to produce the final product.
Current production methods for fastening components to a plastic part of low ductility include forming a threaded recess in the plastic part, which serves as a base, and into which a threaded member or fastener is screwed. This has been found to be undesirable in several respects. The brittle nature of polycarbon and other plastics employed makes it difficult to cut threads in the polycarbon base for receiving the threaded member. During thread formation, the brittle polycarbon base material has been found to crack and chip, often making insertion of the threaded fastener difficult or impossible. Also, the chipping reduces the bearing surface area against which the threads of the fastener bear against the base to preclude the fastener from being pulled out of the base. Thus, the load bearing capacity of the threaded member is significantly reduced. bearing capacity of the threaded member is significantly reduced.
Due to the aforementioned difficulties associated with pre-threading brittle plastic base materials, other, less desirable, fastening means have been employed. One such alternative fastening means attempted has been to employ a self-threading screw. However, the brittle nature of the plastic base material precludes the formation of a consistent thread therein, with a resultant cracking of the base.
Alternatively, internally threaded metal inserts have been molded to the base, with the threaded fastener then screwed into the internal threads of the metal insert. However, due to the inherent incompatibility of metals and plastics, the bond therebetween has been found inadequate to maintain bonding of the metal insert to the plastic base when the metal insert is pulled with significant force. Bonding of the metal insert is attained by providing the metal insert with grooves, ridges or knurls and bringing the base plastic to a flowable state whereby the base plastic flows into the grooves, ridges or knurls of the insert. Accordingly, the strength of the bond is only derived from the base plastic, and limited thereby. Hence, upon imposition of significant forces pulling on the metal insert, the insert is pulled from the base, together with any threaded member screwed into the insert. Additionally, formation of the metal inserts adds considerable expense to the overall production cost.
One method currently employed for fastening components to the base which provides the desired bonding strength is a conventional screw and nut assembly. However, this is undesirable in several respects. Functionally, the screw and nut assembly loosens over time, resulting in a loose connection between components. Also, aesthetically, screw and nut assemblies are undesirable due to the requirement of a provision for accommodating the nut or the nut being exposed on one end of the base, rather than allowing for employment of a blind tapping which provides superior aesthetics. Furthermore, the requirement of the metal nut adds undesirable cost to production.
There is a need for eliminating the driving of a metal fastener into the plastic base. This operation of driving the metal fastener requires assembly time and the use of labor. Overall production efficiency could be obtained by eliminating a tapping of a thread into a plastic base and/or the driving of a fastener into the thread in the plastic base. In its preferred form the invention allows elimination of the driving of the fastener into the plastic base thereby increasing overall efficiency in mounting components to a plastic base with a metal threaded fastener.
Also, there is a need to join together plastic bodies such as two plastic components or a pair of plastic sheets or plates with one or more plastic fasteners. While metal fasteners may be used as above-described, there are times when it is desired not to use metal and still join plastic parts or components together to form a composite of the two joined parts. In such instances, it is preferred that the plastic fastener form a strong, secure connection that will not readily loosen as may a plastic screw that is threaded into the parts to join them together. Preferably, the plastic fastener should bond the two plastic bodies together much in the manner that of an integral bond therebetween.
The present invention is also of particular use to molders of plastic parts, such as molded plastic shells or molded plastic bodies that have cylindrical protrusions or bosses thereon to create an attachment area for a screw or other fastening means to secure one body to another body or to a supporting frame, or to attach other elements to the molded plastic body. For example, in the automotive industry, dashboards are molded from plastic with an exterior shell that has the same outside appearance but on the inside, has different bosses to receive screws or other fasteners to secure different instruments which vary depending on the model and/or the instrument options selected by the purchaser. Currently, the automotive company and the molder have one mold for each of the options; and this may result in twenty or more molds to accommodate each internal change of the bosses or reinforcing thick portions on the plastic shell body which has the same outward appearance.
The injection molding of these thick portions or bosses is a time-limiting factor on the molding process because the time needed for the plastic to solidify before part ejection is usually determined by the thickness of the plastic cross-section, and it takes longer to solidify the thicker bosses than the thin shell cross-section.
Also, in a mold, the boss configuration is the highest wear part of the molding die. A pin which is commonly called a core pin is inserted in the center of the boss to create an attachment area for a screw or other method of attaching the shell. This pin is nominally tapered and must be closely controlled because of the amount of pressure and friction the pin must withstand. The pin is normally the highest wear point of the mold and requires the most rework and replacement.
Additionally, strength requirement for the boss may determine what plastic is used for the shell because the strength needed for the bosses used for affixing the shell to another shell or to another part or frame or to mount other elements may outweigh other considerations. That is, the product may be molded with more or larger bosses or higher strength plastic materials than the shell requires in order to provide the strength to the bosses to interconnect the shell with other parts. Additionally, some products that are now molded with integral plastic bosses, e.g., integrated circuit boards or the like, could be made less expensively from plastic flat sheet stock with subsequently attached bosses.
From the foregoing, it will be seen that there is a need for a new and improved method for forming plastic products with integrally attached plastic projections such as bosses.